The present invention relates to a method for setting-up a connection in a radio communications system which includes at least one first radio station having at least one transmitter/receiver, wherein the radio communications system has access to channels that are intended for connections between at least one first subscriber and at least one further radio station, and wherein the properties of the connections are contingent on system parameters. The invention also relates to handover procedures, optimization of channel utilization and channel changes, and also relates to an arrangement for carrying out these methods and procedures.
When setting-up connections between two subscribers in a mobile telephone system for instance, two-directional connections are established between mobile stations and base stations. Each connection includes two channels, of which one is used for communication from one of the mobile stations to one of the base stations, and the other channel is used for communication in the opposite direction. It can be established whether or not the properties of a connection are sufficiently good for two-directional communication, by measuring parameters which reflect the properties of the connection.
One method of determining the properties of a connection between a base station and a mobile station is to calculate the C/I-value of the connection. This can be effected both in the uplink, i.e. the connection from the mobile station to the base station, and in the downlink, i.e. the connection from the base station to the mobile station. In the case of the uplink, the C/I-value is the quotient between the signal strength C of a mobile station detected by a receiving base station and the sum of the signal strengths of interfering mobile stations in the local cell or in neighbouring cells detected by said base station. Thus, the C/I-value is a measurement which is used as a parameter in determining whether or not the properties of a connection are sufficiently good.
A channel can be allocated a connection in a known manner when the channel fulfills a predetermined criterion. In this regard, one method of channel allocation is to estimate the C/I-value of all channels, by assuming a given transmission power, among other things. The criterion for channel allocation is to allocate the channel that has the highest C/I-value. One drawback with this method is that the channel is not allocated on the basis of the actual need of the mobile, and instead the mobile is allocated the best channel, meaning that resources are not optimized. This problem is considered in "The Third IEEE International Symposium on Personal, Indoor and Mobile Radio Communications", Oct. 19-21, 1992, Boston, Mass., U.S.A. in the conference report "Novel Radio Access Principles Useful for Third Generation Mobile Radio Systems" by Dag Akerberg, Ericsson Radio Systems AB.
Another criterion is that the C/I-value shall exceed a given threshold value and that the channel allocated is one which just exceeds the threshold value. This is illustrated in the IEEE article "Reduction of Cochannel Interference in Cellular Systems by Intra-Zone Channel Reassignment and Adaptive Transmitter Power Control" by Teruya Fuji and Masayuki Sakamoto, NTT, Japan.
The European Patent Application EP 0,419,205 teaches a channel allocation method for application in a mobile telephone system which includes mobile stations and base stations. A speed detector measures the speed of the mobile stations and a signal generator generates a speed dependent value. A channel which exhibits quality properties that agree with the value generated by the signal generator is allocated to respective mobile stations. Channel selection is thus influenced by the speed at which the mobile stations move.
All of the aforedescribed methods have the common drawback of solely taking one or a few parameters into account when assessing a connection. The algorithms required to fulfil the set criteria are also complicated and, in many instances, difficult to perform.
A general problem related to mobile telephone systems is that the allocation of two-directional connections between base stations and mobiles is effected without selecting channels in an adequate manner. When selecting channels, attention is paid only to a few parameters, for instance only to the C/I-value, when deciding whether or not the connection is satisfactory. This has resulted in the system allocation of a channel which has good quality properties for a connection without taking into account the consequences of this allocation, for instance consequences in the form of interference on other connections or of uneven distribution of the radio resources between different base stations. Consequently, the mobile telephone system has not been optimized with respect, for instance, to radio resources and interference that is liable to affect other connections. Examples of radio communications systems in which the aforesaid problems occur are mobile telephone systems which use frequency division multiple access (FDMA), time division multiple access (TDMA), coding (CDMA) and so-called DECT-systems.